Vintage DB pickups - how´d it start?

Mostly for my own personal amusement I´ve tried to research the history and development of double bass amplification. I am very well familiar with the Ampeg pickup from the 40s, and the DeArmond that came in the early 50s. I´d like to get in touch with people who knows of or have experience with other older DB pickups. There were a number of magnetic units, such as the Schaller which is still sold! I believe that many of the pickup systems were sold nationally or regionally only. In France the Stimer Co offered a pickup similar to the Schaller (they also made pickups for Django-type guitars), and in Sweden where I live, Hagström/Bjärton offered some kind of DB pickup already in 1952. Don´t know how it worked, though...